Various MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing, and wherein a substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
FIGS. 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 103 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 102 is mechanically coupled to a generally rigid structural layer or backplate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 102 is embedded within the backplate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer, which may have upper and lower oxide layers 106, 107 formed thereon. A cavity or through-hole 108 in the substrate and in any overlying layers (hereinafter also referred to as a substrate cavity) is provided below the membrane, and may be formed for example using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110.
A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
A further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged in the back-plate 104 so as to allow free movement of air molecules through the back plate, such that the second cavity 110 forms part of an acoustic volume with a space on the other side of the back-plate. The membrane 101 is thus supported between two volumes, one volume comprising cavities 109 and substrate cavity 108 and another volume comprising cavity 110 and any space above the back-plate. These volumes are sized such that the membrane can move in response to the sound waves entering via one of these volumes. Typically the volume through which incident sound waves reach the membrane is termed the “front volume” with the other volume, which may be substantially sealed, being referred to as a “back volume”.
In some applications the backplate may be arranged in the front volume, so that incident sound reaches the membrane via the acoustic holes 112 in the backplate 104. In such a case the substrate cavity 108 may be sized to provide at least a significant part of a suitable back-volume.
In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use, i.e. the substrate cavity forms part of an acoustic channel to the membrane and part of the front volume. In such applications the backplate 104 forms part of the back-volume which is typically enclosed by some other structure, such as a suitable package.
It should also be noted that whilst FIG. 1 shows the backplate 104 being supported on the opposite side of the membrane to the substrate 105, arrangements are known where the backplate 104 is formed closest to the substrate with the membrane layer 101 supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium position. The distance between the lower electrode 103 and the upper electrode 102 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown). The bleed holes allow the pressure in the first and second cavities to equalize over a relatively long timescale (in acoustic frequency terms) which reduces the effect of low frequency pressure variations, e.g. arising from temperature variations and the like, but without significantly impacting on sensitivity at the desired acoustic frequencies.
One skilled in the art will appreciate that MEMS transducers are typically formed on a wafer before being singulated. Increasing it is proposed that at least some electronic circuitry, e.g. for read-out and/or drive of the transducer, is also provided as part of an integrated circuit with the transducer. For example a MEMS microphone may be formed as an integrated circuit with at least some amplifier circuitry and/or some circuitry for biasing the microphone. The footprint of the area required for the transducer and any circuitry will determine how many devices can be formed on a given wafer and thus impact on the cost of the MEMS device. There is therefore a general desire to reduce the footprint required for fabrication of a MEMS device on a wafer.
In addition to be suitable for use in portable electronic devices such transducers should be able to survive the expected handling and use of the portable device, which may include the device being accidentally dropped.
If a device such as a mobile telephone is subject to a fall, this can result not only in a mechanical shock due to impact but also a high pressure impulse incident on a MEMS transducer. For example, a mobile telephone may have a sound port for a MEMS microphone on one face of the device. If the device falls onto that face, some air may be compressed by the falling device and forced into the sound port. This may result in a high pressure impulse incident on the transducer. It has been found that in conventional MEMS transducers high pressure impulses can potentially lead to damage of the transducer.
FIG. 2 illustrates a cross-sectional view through a typical transducer structure. The transducer structure comprises a membrane 101 which is moveable during use in relation to a rigid backplate 104. The membrane 101 and backplate 104 are supported by a substrate 105, the substrate 105 comprising a cavity or through-hole 108. Electrodes and other features are not shown in FIG. 2 for clarity purposes.
Referring to FIG. 3, during movement of the membrane 101 during use, and in particular during high input acoustic pressure, or extreme conditions such as a mobile device being dropped, it is possible that the membrane 101 makes contact with the substrate 105 which provides support for the membrane. For example, the membrane 101 can make contact with a peripheral edge of the substrate 105 that forms the cavity within the substrate, as illustrated by the arrow 30. This can result in the membrane becoming damaged.
This problem may be particularly apparent in transducer configurations—such as is illustrated in FIG. 4—having a generally square-shaped membrane layer wherein the membrane layer comprises an active central region 301 and a plurality of support arms 303 which extend laterally from the active central region for supporting the active central region of the membrane. In this case it will be appreciated that the edges of the supporting arms 303 may be particularly vulnerable to damage if the supporting arms of the active membrane area make contact with the edge 308 of the substrate cavity.
Furthermore, the occurrence of membrane stiction—whereby the membrane becomes permanently or temporarily adhered to the substrate—may also be observed following a high pressure event which causes the membrane to make contact with the substrate. This is illustrated in FIGS. 5a and 5b. Specifically, in FIG. 5a the membrane 101 is suspended freely with respect to the substrate 105. However, in FIG. 5b the membrane 101 has become adhered to the upper surface of the substrate 105 following e.g. a high pressure event. It will be appreciated that stiction arises when e.g. atomic-level attractive forces and/or capillary forces and/or chemical bonding arising between the membrane and the substrate exceed restoring forces e.g. arising from the elasticity of the membrane which act to restore the membrane to an equilibrium position. Membrane stiction may significantly degrade the performance of the transducer or may even result in the failure of the transducer.